Table of Contents

HashMap in Java with Examples

只允許 1 個 Null Key 鍵值，不保證依照新增順序保持排序，是一個非同步的操作，HashMap in Java 介紹常見的 containsKey 、 replace 、 entrySet 、 forEach 、 toArray 等方法，本篇增加了範例，並透過單元測試來驗證產出結果。

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- map
       |                   +- hashmap
       |                       +- HashMapWithExamplesTest.java

單元測試

HashMap Java 提供循環訪問、轉成陣列等操作 Map 中的元素。

entrySet

建立一個 HashMap ，內有三個元素，迴圈取得元素。

	@Test
	public void entrySet() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		for (Map.Entry<String, Integer> e : map.entrySet()) {
			System.out.println(e.getKey() + ", " + e.getValue());
		}
	}

Grape, 1
Kiwifruit, 2
Lemon, 3

forEach

Java HashMap 建立一個 HashMap ，內有三個元素，迴圈取得元素。

	@Test
	public void forEach() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.forEach((k, v) -> System.out.println(k + ", " + v));
	}

Grape, 1
Kiwifruit, 2
Lemon, 3

keyForEach

HashMap Java 建立一個 HashMap ，內有三個元素，迴圈取得 key 元素。

	@Test
	public void keyForEach() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.keySet().forEach(e -> System.out.println(e));
	}

Grape
Kiwifruit
Lemon

keyForEachRemaining

HashMap Java 建立一個 HashMap ，內有三個元素，迴圈取得 key 剩餘元素。

	@Test
	public void keyForEachRemaining() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Set<String> set = map.keySet();
		Iterator<String> it = set.iterator();
		int i = 0;
		while (it.hasNext()) {
			System.out.println(it.next());
			if (i == 1) {
				break;
			}
			i++;
		}
		System.out.println("----------");
		it.forEachRemaining(e -> {
			System.out.println(e);
		});
	}

Grape
Kiwifruit
----------
Lemon

keyIterator

Java HashMap 建立一個 HashMap ，內有三個元素，迴圈取得 key 元素。

	@Test
	public void keyIterator() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Iterator<String> it = map.keySet().iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}

Grape
Kiwifruit
Lemon

valueForEach

Java HashMap 建立一個 HashMap ，內有三個元素，迴圈取得 value 元素。

	@Test
	public void valueForEach() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.values().forEach(System.out::println);
	}

1
2
3

valueForEachRemaining

Java HashMap 建立一個 HashMap ，內有三個元素，迴圈取得 value 剩餘元素。

	@Test
	public void valueForEachRemaining() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Collection<Integer> collection = map.values();
		Iterator<Integer> it = collection.iterator();
		int i = 0;
		while (it.hasNext()) {
			System.out.println(it.next());
			if (i == 1) {
				break;
			}
			i++;
		}
		System.out.println("----------");
		it.forEachRemaining(e -> {
			System.out.println(e);
		});
	}

1
2
----------
3

valueIterator

Java HashMap 建立一個 HashMap ，內有三個元素，迴圈取得 value 元素。

	@Test
	public void valueIterator() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Iterator<Integer> it = map.values().iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}

1
2
3

keyToArray

建立一個 HashMap ，內有三個元素， key 轉換成 String 陣列。

	@Test
	public void keyToArray() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);

		String[] array = new String[map.size()];
		map.keySet().toArray(array);
		for (String e : array) {
			System.out.println(e);
		}
		assertEquals(expectedSize, array.length);
	}

Grape
Kiwifruit
Lemon

keyStreamToArray

建立一個 HashMap ，內有三個元素，使用 stream ， key 轉換成 String 陣列。

	@Test
	public void keyStreamToArray() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);

		String[] array = map.keySet().stream().toArray(String[]::new);
		for (String e : array) {
			System.out.println(e);
		}
		assertEquals(expectedSize, array.length);
	}

Grape
Kiwifruit
Lemon

HashMapWithExamplesTest.java

HashMap in Java 新增單元測試，驗證是否符合預期。

package org.ruoxue.java_147.map.hashmap;

import static org.junit.Assert.*;

import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

import org.junit.Test;

public class HashMapWithExamplesTest {

	@Test
	public void entrySet() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		for (Map.Entry<String, Integer> e : map.entrySet()) {
			System.out.println(e.getKey() + ", " + e.getValue());
		}
	}

	@Test
	public void forEach() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.forEach((k, v) -> System.out.println(k + ", " + v));
	}

	@Test
	public void keyForEach() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.keySet().forEach(e -> System.out.println(e));
	}

	@Test
	public void keyForEachRemaining() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Set<String> set = map.keySet();
		Iterator<String> it = set.iterator();
		int i = 0;
		while (it.hasNext()) {
			System.out.println(it.next());
			if (i == 1) {
				break;
			}
			i++;
		}
		System.out.println("----------");
		it.forEachRemaining(e -> {
			System.out.println(e);
		});
	}

	@Test
	public void keyIterator() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Iterator<String> it = map.keySet().iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}

	@Test
	public void valueForEach() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		map.values().forEach(System.out::println);
	}

	@Test
	public void valueForEachRemaining() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Collection<Integer> collection = map.values();
		Iterator<Integer> it = collection.iterator();
		int i = 0;
		while (it.hasNext()) {
			System.out.println(it.next());
			if (i == 1) {
				break;
			}
			i++;
		}
		System.out.println("----------");
		it.forEachRemaining(e -> {
			System.out.println(e);
		});
	}

	@Test
	public void valueIterator() {
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);
		Iterator<Integer> it = map.values().iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}

	@Test
	public void keyToArray() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);

		String[] array = new String[map.size()];
		map.keySet().toArray(array);
		for (String e : array) {
			System.out.println(e);
		}
		assertEquals(expectedSize, array.length);
	}

	@Test
	public void keyStreamToArray() {
		int expectedSize = 3;
		Map<String, Integer> map = new HashMap<String, Integer>();
		map.put("Grape", 1);
		map.put("Kiwifruit", 2);
		map.put("Lemon", 3);

		String[] array = map.keySet().stream().toArray(String[]::new);
		for (String e : array) {
			System.out.println(e);
		}
		assertEquals(expectedSize, array.length);
	}
}